Select the cell or range where you want the dropdown
Go to the Data tab
Click Data Validation
In the Allow box, choose List
In the Source box, type the items separated by commas or select the cell range containing the list
Make sure the In-cell dropdown box is checked
Click OK
Click the cell to use the dropdown list
To edit the dropdown, go back to Data Validation and change the source list
To copy the dropdown to other cells, use copy and paste or drag the fill handle